HumidityMeasurementTechnology– pros&cons

Helping you make a better measurement.

BruceMcDuffee MichaelBoetzkes

WebinarPresenters&HumidityExperts

Agenda&Takeaways

Agenda• Parameterreview• Mechanical• Psychrometer (wet-bulb)• CapacitiveSensors• ResistiveSensors• Chilledmirror• Metaloxide• Applicationcases

Takeaways• Knowwhichsensormakessenseforyourapplication.

Parameteroverview

• RelativeHumidity• Vaporpressure• Dewpoint• Wetbulb

Mechanical

• Usesorganicmaterialsthatchangeinshapeduetochangesinrelativehumidity.

• Shapechangeisconnectedtomechanicallevers,springs,etc.tooutputareading

• Humanhair,catgut,textile,goldbeater’sskin

Mechanical

Pros Cons

Noelectricityneeded Notreliable

Fun Impossible tocalibrate

Onlyaccurate midrange(30to80%RH)

High inhysteresis

Veryslowresponse

Use a mechanical when:• When you need a general idea• If you don’t have much money to spend on humidity measurement• For fun if you’re a humidity geek

Image source - Wikipedia

Psychrometer (wet-bulb)

• Requires2temperaturesensors.• Oneiscoveredwithwetsock.• Airisdrawnoverthetemperaturesensor.

• Thedifferenceordepressionbetweenthetwomeasurementsindicatestheamountofwatervaporpresent. Image source: Wikipedia

Psychrometer (wet-bulb)Pros Cons

Inexpensive($30to$90onAmazon) Requires skillandconsistencytooperatewell

Noproblemwithcondensation Notaccurateinsmallspaces

Easyconversionwithcharts orgraphs Manyvariables=highuncertainty

Use a psychrometer when:• Working with evaporative cooling installations• Large air sample is available• Able to control the variables• Operators remain consistent• No electricity is available (and don’t want to rely on or carry batteries)

Image source: commons.Wikimedia.com

CapacitiveSensors

• Asrelativehumiditychanges,theamountofwatermoleculesabsorbedbythepolymerchangesthedielectricconstantofthecapacitor,varyingthecapacitance.

• Astherelativehumidityincreases,thecapacitanceincreasesalmostinalinearfashion.

Electronic Humidity Sensor

Porous

electrode

Polymeror oxidedlelectriclayer

Electrode

CapacitiveSensorsPros Cons

Accurate andreliable Limitedaccuracy inverydryconditions

Resilienttowater, dustandchemicals Dependentonthemanufacturer

Smallsize Requireselectronicsto generatethesignal

Fast response Hightemp+highRH canaffectcalibration

Widemeasurementrange(0to100%RH) Moreexpensive thanresistive

Widetemperaturerange(-70to+200C)

Use a capacitive sensor when:• Condensation is possible• Dirty environment• Need good performance (accuracy, repeatability, stability, etc.)

ResistiveSensors

• Asrelativehumiditychanges,theresistancechanges.

• IncreaseinRHcauseslowerresistance.

Image source: Sensor Magazine

ResistiveSensors

Pros Cons

Fieldreplaceablewithoutcalibration Notsuitableforextremes

Inexpensive Notresistanttoharsh conditions

Goodformid-rangeRH(20%to90%) Repeatabilityquestionable

Cannot toleratecondensation

Use a resistive sensor when:• conditions are moderate• 20% - 90% RH• Condensation is not expected to occur• Accuracy and repeatability are not critical

Image source: Sensor Magazine

ChilledMirror

• Fundamentalmeasurement• Dewpointtemperatureisdeterminedbythetemperatureofthemirrorwhendeworfrostformsonthemirror.

• Beawareofdewpointvsfrostpoint.• Knowthepressureatthepointofinterest.

Image source: Thunder Scientific

ChilledMirrororCondensationHygrometer

Pros Cons

Lowuncertainties;(+/- .1Ck=2) Expensive comparedtoothertechnologies

Highlystable Requiresverycleanenvironment

Broadrange(-100Cto+100C) High levelofmaintenance

Fundamentalmeasurement Requiresaskilled, trainedoperator

Airflow throughsensinghead

Use a chilled mirror when:• Low uncertainty and high stability are a requirement (Lab, R&D)• Measurement environment is clean and controlled• You have a lot of money and you don’t know what to do with it.• Not recommended for inline or field applications.

MetalOxide

• AluminumOxide(Al2O3)mostcommon

• Watervaporadsorbstooxide• Conductivityvarieswithamountofwatervapormolecules

• Givesanabsolutemeasure• Needpressureandtemperatureforconversions

Image source: Sensor Magazine

MetalOxide

Pros Cons

Excellentforverydryconditions(aslowas-100C)

Slowresponse tochanges

Very smallfootprint Largecalibrationshiftsfromwetair

Requires numerouscalibrations

Calibrationisusuallyveryexpensive

Notstableovertime

Use a metal oxide when:• Very dry, very clean air/gas• For dew point temperature < -70C• Stable, uneventful air streams• Measuring trace moisture in absolute terms (ppm, mixing ratio, p)

SummaryofTechnologies

Othertechnologies:• Spectroscopy• Colorchangingchemicals• Saturatedlithiumchloride• Electrolytic• Acoustic• Adiabaticexpansion• Gravimetric• Andmore

1. Mechanical2. Psychrometer (wet-bulb)3. CapacitiveSensors4. ResistiveSensors5. Chilledmirror6. Metaloxide

Comments&Questions

Pleasetypeyourquestionsintothechatboxatthelowerleftportionofyourscreen.

TakeawaysforaBetterMeasurement

ChoosetheBestTechnologyforYourApplication

Glovebox whereyouhavetomaintain5ppmwatervapor.

• Temperatureis23C• Airisveryclean• Minimalchanceofcondensation• Needveryhighaccuracy,reliabilityandstability.

• Moneyisnoobject

Image source: Flickr: Glovebox; Author: Idaho National Laboratory

ChoosetheBestTechnologyforYourApplication

• Compressedairsystem• Desiccantdryers-40Cdewpoint• 90psigoperatingpressure• Industrialenvironment• Condensationlikelyatsomepointinthesystem(unreliabledryers)

• Needinlinemeasurementatcriticalpointsofuse

• Lowbudgetoperation

ChoosetheBestTechnologyforYourApplication

Cleanroom• Temperatureis23C• Humidity40%• ParticlesizeClassISO3• Airpressuredifference• Airchangerate• Airflowvelocity

SummaryofChoosingtheCorrectInstrument

1. Measurementenvironment2. Performancerequirements3. Parameterrequired

Comments&Questions

Ifwedon’tgettoyourquestiontoday,we’llrespondviaemailafterthewebinar.

Rotronic Academy

• Resourcesforhumiditymeasurement• ResourcesforcGMPmapping• Resourcesforenvironmentalmonitoring

www.rotronic-usa.com/academy

NextWebinarHowtoMapaWarehouseThursday,December1st1:00PMESTwww.rotronic-usa.com/webinars

Helpingyoumakeabetterhumiditymeasurement– andmore.

Transmitters,meters,OEMprobesforhumidity,dewpoint,CO2,differentialpressure,andmore.

NEW!Rotronic MonitoringSystemwww.rotronic-usa.com/RMS

Thankyou!email:info@rotronic-usa.comRotronic:www.rotronic-usa.com

On-demandwebinarsandregistrations:www.rotronic-usa.com/webinars